Question

The area of a circle with radius r is given by the formula A=πr2. If a circle has area 81π square inches, what is the radius (in inches)

Answer 1

The Solution:

GIven:

$A=\pi r^2$

Required:

$Find\text{ }r\text{ when }A=81\pi in^2$

Substitute:

$\begin{gathered} \pi r^2=81\pi \\ \\ r^2=\frac{81\pi}{\pi}=81 \end{gathered}$

Take the square root of both sides:

$\begin{gathered} r=\sqrt{81}=\pm9 \\ \\ r=9in\text{ \lparen discard r=-9 since }r>0) \end{gathered}$

Answer:

9 inches